Calabrese — a privacy lobbyist — was first careful to note that the ACLU doesn't strictly oppose universal background checks for gun purchases. "If you're going to require a background check, we think it should be effective," Calabrese explained.

"However, we also believe those checks have to be conducted in a way that protects privacy and civil liberties. So, in that regard, we think the current legislation, the current proposal on universal background checks raises two significant concerns," he went on.

"The first is that it treats the records for private purchases very differently than purchases made through licensed sellers. Under existing law, most information regarding an approved purchase is destroyed within 24 hours when a licensed seller does a [National Instant Criminal Background Check System] check now," Calabrese said, "and almost all of it is destroyed within 90 days."

Calabrese wouldn't characterize the current legislation's record-keeping provision as a "national gun registry" — which the White House has denied pursuing — but he did say that such a registry could be "a second step."

"[U]nfortunately, we have seen in the past that the creation of these types of records leads sometimes to the creation of government databases and collections of personal information on all of us," Calabrese warned. "That's not an inevitable result, but we have seen that happen in the past, certainly."

"As we've seen with many large government databases, if you build it, they will come."

"And existing law also bars the use of those records for other purposes," Calabrese continued, explaining that the government is supposed to be barred by the Privacy Act from transferring database information between agencies without the consent of the individual citizen.

"We think those are privacy best practices," Calabrese said. "We think almost all government databases should operate that way."

"Once you no longer need the information, you should destroy it. Information collected for one purpose shouldn't be used for another purpose," he said.

But Calabrese says that Reid's legislation fails to include those "privacy best practices."

"Contrast this with what the existing [Reid] legislation says, which is simply that a record has to be kept of a private transfer," Calabrese highlighted, "and it doesn't have any of the protections that we have in current law for existing licensees."

"We think that that kind of record-keeping requirement could result in keeping long-term detailed records of purchases and creation of a new government database."

"And they come to use databases for all sorts of different purposes," Calabrese said. "For example, the National Counterterrorism Center recently gave itself the authority to collect all kinds of existing federal databases and performed terrorism related searches regarding those databases. They essentially exempted themselves from a lot of existing Privacy Act protections."

"So you just worry that you're going to see searches of the databases and an expansion for purposes that were not intended when the information was collected."

Reid's legislation is hauntingly vague about who would physically keep information about American gun purchases, but it's crystal clear that records will be kept.

"Regulations … shall include a provision requiring a record of transaction of any transfer that occurred between an unlicensed transferor and unlicensed transferee," according to the bill.

The ACLU's second "significant concern" with Reid's legislation is that it too broadly defines the term "transfer," creating complicated criminal law that law-abiding Americans may unwittingly break.

"[I]t's certainly a civil liberties concern," Calabrese told TheDC. "You worry about, in essence, a criminal justice trap where a lawful gun owner who wants to obey the law inadvertently runs afoul of the criminal law."

"They don't intend to transfer a gun or they don't think that's what they're doing, but under the law they can be defined as making a transfer. We think it's important that anything that is tied to a criminal sanction be easy to understand and avoid allowing too much prosecutorial discretion."

"For example, different gun ranges are treated differently," Calabrese said. "You're firing a firearm in one geographic location, you're OK, but in another, you're not. And those kind things, it's going to be hard for your average consumer to really internalize and figure out the difference."

"Criminal sanctions shouldn't hinge on those kinds of differences," he said.

Separate from the ACLU's concerns with a universal background check system, Calabrese flagged another provision of the legislation invented by Sen. Boxer that the ACLU is "worried about" — school tiplines for the reporting of "potentially dangerous students"

"We're worried about this tip line," Calabrese admitted. "We think we already have a phone number for reporting dangerous situations — it's called 9-1-1."

"The tip line doesn't have any guidance for who should be included, how we should vet these requests, who should be included in the system, what you should do with this information once you get it," he warned. "It just seems like a dangerously unregulated avenue that's going to risk pushing more kids into the criminal justice system."

"What's a school supposed to do if they get an anonymous phone call that some kid is dangerous?" Calabrese went on. "How are they supposed to treat that? Do they have liability if they ignore it? Should this kid be suspended? Or should he be scrutinized by a school safety officer because of an anonymous tip?"

"You could see how this could run amok very quickly. These are high schools. Lord knows, if you're going to give a kid an anonymous opportunity to lash out at someone, you're going to see a lot of problems."